A compressed air supply device is used in vehicles of all types, in particular, to supply compressed air to a vehicle air suspension installation. Air suspension installations may comprise level control devices, with which the spacing between the vehicle axle and the vehicle body can be adjusted. An air suspension installation comprises a number of air bellows, which are pneumatically connected to a common line (i.e., a gallery), can lift the vehicle body when increasingly filled, and can accordingly lower the vehicle body when decreasingly filled. With growing spacing between the vehicle axle and the vehicle body or ground clearance, the suspension travels are longer, and greater ground unevenness can also be overcome without resulting in contact with the vehicle body. Such systems are used in off-road vehicles and sport utility vehicles (SUVs). In particular, in the case of SUVs with powerful engines, it is desirable to provide the vehicle with relatively small ground clearance for high speeds on the road and also with a relatively large ground clearance for off-road. It is also desirable to change the ground clearance as quickly as possible, which increases the demands, in terms of speed, flexibility, and reliability of a compressed air supply device.
A compressed air supply device for use in a pneumatic system comprising a pneumatic installation (for example, an air suspension installation as previously described) is operated with compressed air from a compressed air supply unit, for example, within the scope of a pressure level from 5 to 20 bar. The compressed air is made available to the compressed air supply unit by means of an air compression unit (e.g., a compressor). The compressed air supply unit is pneumatically connected to a compressed air port in order to supply the pneumatic installation and is also pneumatically connected to a bleeding port. The compressed air supply device an be bled toward the bleeding port via a bleeding valve arrangement by draining air.
To ensure long-term operation of the compressed air supply device, the device has an air dryer, with which the compressed air is to be dried. The collection of moisture in the pneumatic system is thus avoided, which, at comparatively low temperatures, may lead to valve-damaging crystal formation and undesired defects in the compressed air supply device and in the pneumatic installation. An air dyer has a drying means, normally a granulate packed bed, through which the compressed air can flow so that the granulate packed bed (at comparatively high pressure), by means of adsorption, can take on moisture contained in the compressed air. An air dryer may optionally be designed as a regenerative air dryer. This is achieved as a result of the fact that, during each bleeding cycle (at comparatively low pressure), the dried compressed air from the air suspension system flows through the granulate packed bed in counter flow or co-current flow relative to the filling direction. The bleeding valve arrangement can be opened for this purpose. For such an application (also referred to as pressure swing adsorption), it has proven to be desirable to design a compressed air supply device in a versatile and reliable manner, in particular, to enable relatively quick bleeding with a pressure swing that is still sufficient for regeneration of the air dryer.
A compressed air supply device of the type mentioned above for a vehicle level control device and air suspension systems is known (see DE 35 429 74 A1), with which a predefined spacing of the vehicle frame from the vehicle axle can be maintained in accordance with the vehicle load by filling up or emptying the air suspensions. The device contains a normally closed solenoid bleeding valve and a safety valve controllable by the pressure in the air suspensions. Such a compressed air supply device can be improved further.
DE 199 11 933 B4 describes a compressed air generator comprising an air dryer with a first compressed air supply line, wherein the compressed air is guided through a drying agent, and comprising a second compressed air supply line without guiding the compressed air through the drying agent.
A compressed air supply device as mentioned above is also disclosed in EP 1 165 333 B2 within the scope of a pneumatic system comprising an air suspension installation. Besides a main bleeding line that can be shut off separately, the compressed air supply device has a high-pressure bleeding line, which has a high-pressure bleeding valve in addition to a main bleeding valve in the main bleeding line controlled pneumatically by a control valve, and which is connected in parallel to the main bleeding line. The free flow cross section of the separate high-pressure bleeding valve is smaller than that of the main bleeding valve. Such a compressed air supply device can be improved further. It has been found that, when bleeding such a compressed air supply device via the high-pressure bleeding line, dry air is bled, which is not used for the regeneration of the drying agent. This amounts to an unnecessary waste of dry air, in particular, for the case in which a flexible, quick, yet reliable actuation of the compressed air supply device suitable for above applications should be necessary with correspondingly high actuation rates.
EP 1 233 183 B1 describes a compressed air control device comprising an air dryer. The housing of the air dryer includes a pot-shaped drying container, the interior of which can be connected via the housing to a pressure medium source and also to a port element in the form of a pressure accumulator and/or an air suspension via a valve that is open to the port element. The housing has an air inlet and an air outlet for the compressed air, which, in order to fill the at least one port element, is guided in a direction of flow from the air inlet, through the drying container, and to the air outlet and, for emptying, is discharged from the housing in an opposite direction of flow from the air outlet and through the drying container and the housing. A controllable directional valve is integrated and installed in the housing of the air dryer and, during emptying, is used to inlet the air into the housing interior and the drying container. A directional valve controlling the discharge duct is controlled by at least one further controllable directional valve by means of the pressure during emptying, wherein this solenoid valve arrangement is arranged substantially outside the housing of the air dryer.
An air dryer of a compressed air device, which can be charged by a compressor, and which has a compressed air storage container and an outlet valve, is also known (see DE 32 16 329 C2). In such an air dryer, a pressure retention valve is provided in a return line between the compressed air storage container on the one hand and the container containing a drying agent as well as the outlet valve on the other hand. The outlet valve and a pressure regulator controlling the outlet valve are connected to a hollow rod by means of the container of the air dryer containing the drying agent in such a way that air can pass through. Such solutions and other previously known solutions for an air dryer have proven to be relatively bulky or require much installation space. It is desirable to design a compressed air supply device having a bleeding valve arrangement and an air dryer in a manner that is as space-saving as possible.
In all previously known solutions, in which the bleeding valve arrangement is provided in the form of a controllable solenoid valve arrangement, the pneumatic part of the solenoid valve arrangement is closed in a deactivated state of the magnetic part of the solenoid valve arrangement. That is, the previously known solutions provide a normally closed solenoid valve arrangement. With regard to the solution described in DE 35 42 974 A1, it has been found that a normally closed solenoid bleeding valve arrangement can be disadvantageous since an additional pressure-limiting or safety valve often has to be provided in order to ensure reliable function. Since, in the normally closed state, a relay valve of the solenoid valve arrangement is closed, this may lead in some cases to a valve body adhering to the valve seat, causing the switching function of the relay valve of the solenoid valve arrangement to become unreliable.